Prof. Dr. Ermin Malic from Philipps-Universität Marburg.

Exciton optics, dynamics, and transport in atomically thin semiconductors

Monolayer transition metal dichalcogenides (TMDs) and related van der Waals heterostructures present a new class of atomically thin semiconductors. They exhibit an extraordinarily rich exciton physics including bright and a variety of dark states, spatially separated interlayer and hybrid exciton states as well as moiré-trapped excitons in twisted TMD heterostructures. Based on a density matrix formalism, we perform microscopic modeling of many-particle physics determining exciton optics, dynamics and transport phenomena in these technologically promising materials. In combination with first-principle calculations, our approach is material- specific and has a predictive character.

In joint theory-experiment studies, we reveal that phonon-mediated scattering via hybridized dark excitons governs the ultrafast charge transfer process in TMD heterostructures. We study the phonon-driven relaxation cascade of hot interlayer excitons after the charge transfer has occurred and track the exciton relaxation pathway across different moiré mini-bands and identify the key phonon scattering channels. We unravel a phonon bottleneck in the flat band structure at low twist angles and at low temperatures preventing excitons to fully thermalize into the lowest state explaining the previuosly measured enhanced emission intensity and lifetimes of excited moiré exciton states. We show that counterintuitively flat bands, which typically trap excitons, can significantly enhance exciton propagation due to the emergence of hot excitons. Furthermore, we demonstrate the appearance of highly dipolar in-plane charge-transfer excitons in lateral TMD heterostructures exhibiting a large energy offset at the atomically thin interface. We uncover an intriguing phonon-mediated interplay of the energy-offset-driven exciton drift across the interface and capture into energetically lower-lying charge-transfer excitons at the interface governing their strongly temperature-dependent propagation behaviour.

The gained microscopic insights into the spatiotemporal exciton dynamics are crucial for understanding and controlling technologically important many-particle phenomena in atomically thin semiconductors.